The present invention relates to a technique for reading information from an optical disk and, particularly, a technique for reading information from a phase-change type optical disk on which the information is recorded by utilizing reversible phase-change to be caused by irradiation of the optical disk with light.
It has been known that a phase-change type optical disk has an inherent advantage of easy rewriting without the necessity of an external magnetic field and, therefore, it has been expected that the phase-change type optical disk will become major in the field of write/erase optical disks in the near future.
In the phase-change type optical disk, information is recorded by irradiating a recording film on the disk with a laser light spot having power corresponding to the information, and locally heat the recording film to thereby change phase of the film material between a crystalline state and an amorphous state. Information is reproduced by irradiating the film with laser light of low power to read the difference in reflectance between a crystalline state portion and an amorphous state portion of the recording film as a difference in intensity between reflected light from the crystalline state portion and from the amorphous state portion.
For example, in a phase-change type optical disk utilizing a crystalline recording film whose crystallization time is relatively long, information is recorded on a portion of the recording film formed on the optical disk by irradiating it with laser light to locally increase temperature of the recording film portion to a value higher than the melting point thereof while rotating the optical disk, and by rapidly cooling the melted portion to bring it to an amorphous state.
In erasing the information, the amorphized portion of the recording film is crystallized by heating it to a temperature in a crystallizable temperature range from its crystallization temperature to its melting point and holding the temperature for a time sufficient to crystallize the recording film portion. In this case, an erasing beam to be used has an ellipsoidal cross section whose major axis is in parallel to a moving direction of laser beam on the optical disk.
In a case where a new data is to be written while erasing any previously recorded data, that is, in a case of the so-called pseudo overwriting, the recording film is irradiated with a recording beam having a circular cross section immediately following the erasing beam.
On the other hand, in an optical disk utilizing an information recording film of a material capable of being crystallized at high speed, a single laser beam collimated to a circular cross section is used. In a conventional method, a recording film on an optical disk is crystallized or amorphized by switching laser light between two levels. When the recording film is irradiated with a laser light whose power is large enough to increase its temperature up to its melting point, a portion of the film which is irradiated therewith is amorphized by cooling, while it is crystallized by irradiating it with a laser light whose power is large enough to increase its temperature to a value higher than the crystallization temperature and lower than the melting point and cooling.
The recording film of the phase-change type optical disk is formed of chalcogenide such as GeSbTe, InSbTe, InSe, InTe, AsTeGe, TeOx--GeSn, TeSeSn, SbSeBi or BiSeGe, etc., by using a suitable film forming technique such as resistive heating vapor deposition, electron beam vapor deposition or sputtering, etc. The recording film immediately after formation is in a sort of amorphous state and, therefore, the amorphous state recording film is pre-treated to crystallize it as a whole so that an information recording can be done by locally changing the phase of the recording film to amorphous state by laser beam irradiation.
As mentioned above, in the phase-change type optical disk, data is recorded as an amorphous recording mark on the crystalline recording film. A conventional method of reading the data thus recorded utilizes a difference in reflectance between the amorphous recording mark and the crystalline state portion surrounding the latter and, thus, the thickness of the recording film on the optical disk medium is selected such that the difference in reflectance becomes large enough to clearly distinguish the amorphous portion and the crystalline portion of the recording film.
However, when the difference in reflectance between the crystalline portion and the amorphous portion is made large, thermal absorptance becomes different between these two portions. Thus the power of laser light required for changing from a crystalline state to an amorphous state is different from that required for changing the previous amorphous state to new amorphous state. In such a case, when the overwriting is performed by using a signal beam a variation in the length of the recording mark is produced. Since such variation may cause jitter in a reproduced signal, the quality of the reproduced signal is degraded.
In order to reduce such jitter contained in the reproduced signal, Japanese Patent Application Laid-open No. Hei 2-73537, published on Mar. 13, 1990, proposes to reduce the difference in reflectance between an amorphous portion and a crystalline portion of a recording layer. In such case, however, the level of signal reproduced by using the conventional technique which utilizes the difference in reflectance becomes too small, resulting in a new problem in that the S/N ratio of the reproduced signal is lowered.